DAVIS, Calif. (AP) — Researchers have detected swine flu in elephant seals off the Central California coast, saying it was the first time a human pandemic strain has been found in marine mammals.

However, none of the animals showed clinical signs of the illness.

A University of California, Davis study found the seals contracted the H1N1 virus in 2010, as the pandemic caused by the virus was winding down in humans, the Contra Costa Times reported Wednesday.

The influenza virus commonly crosses species barriers, and it wasn't the first time a marine mammal has been found to carry a human strain, UC Davis professor Tracey Goldstein told the newspaper. However, until now researchers had never found a human pandemic strain in marine mammals, Goldstein said.

Researchers still aren't sure exactly how the seals contracted the virus but said it's unlikely it came from direct contact with people. The researchers raised the possibility that seabirds may have passed on the virus.

"The data suggest the animals were exposed when they were at sea, or coming into the nearshore environment," Goldstein said.

UC Davis wildlife biologists swabbed the noses of 72 elephant seals over the course of two years at Ano Nuevo State Reserve in San Mateo County and Piedras Blancas in San Simeon. The animals were sampled before and after their annual spring foraging trip to Alaskan waters.

The animals tested clean before departure, but two came back bearing the virus in 2010, the newspaper said. A year later, 16 elephant seal pups had blood tests showing they had been exposed to the virus.

Goldstein said the presence of H1N1 in seals is not a cause for alarm.

"It's just a general reminder to people who work with animals to make sure that they protect themselves," she said.

The study was published Wednesday in the online journal PLOS ONE.

The 2009 H1N1 outbreak started a global pandemic that killed as many as half a million people.

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